Data Models and Query Languages

Friday. December 28, 2018 - 1 min

Relational VS Document Databases

• Advantages of Documents Databases:
  • schema flexibility,
  • better performance due to locality,
  • closer to the data structures used by the application
• Advantages of relational model:
  • better support for joins,
  • many-to-one and many-to-many relationships

Schema flexibility in the document model

• Code that reads the data usually assumes some kind of structure, and implicit schema, but not enforced by the database
• schema-on-read vs schema-on-write(relational DB)
• For schema-on-write databases, schema changes are slow
  • MySQL copies the entire table on ALTER TABLE, taking long time

Data locality for queries

Definition: a document is usually stored as a single continuous string.

• If your application often needs to access the entire document, data locality helps.
• If data is split across many tables, multiple index lookups are required, which may require more disk seeks.

Convergence of document and relational databases

• MySQL have supported XML
• PostgreSQL, MySQL and IBM DB2 have supported JSON documents
• RethinkDB supports relational-like joins in its query language
• MongoDB drivers automatically resolve database references